1. Field of the Invention
The invention relates to a method of removing casting defects in articles with oriented microstructure.
2. Brief Description of the Related Art
Single crystal and directionally solidified castings, such as turbine blades, are manufactured using a directional solidification process in which a ceramic shell mould filled with an alloy in the liquid state is withdrawn from a heating zone (temperature above the melting point of the alloy) into a cooling zone (well below the melting point of the alloy in temperature). This is known for example from the documents U.S. Pat. No. 4,96,501, U.S. Pat. No. 3,690,367 or EP-A1-0 749 790. As the shell mould is withdrawn, the liquid alloy solidifies directionally—beginning with that portion of the mould that enters the cooling zone first, and ending with the last portion of the mould to enter the cooling zone. During the solidification of the alloy, the solid/liquid interface is found substantially at a level between the heating and cooling zones. Those skilled in the art of investment casting and directional solidification are aware of the critical importance of maintaining the proper conditions at the solid/liquid interface. For example, too low of a thermal gradient across the solid/liquid interface, and/or too much of an incline of the interface compared to the horizontal, can result in the formation of freckles. Freckles are formed due to interdendritic fluid flow, resulting in chains of equiaxed grains surrounded by material rich in eutectic phases (overly rich in the elements which segregate to the liquid phase during segregation). The chains can be anywhere from 2 mm to 20 cm long, and constitute zones of unacceptable weakness (fatigue strength) in single crystal alloys due to the lack of grain boundary strengtheners. Freckles are also considered critical defects in columnar grain alloys, despite their higher content of grain boundary strengtheners. In other cases, new grains can nucleate and grow for a limited distance in the direction of growth of the solid/liquid interface, provided that the primary orientation (crystallographic orientation relative to the growth direction) is close to that of the rest of the casting. This defect is known as a sliver, and can reach lengths of 5 cm or more. Since it may comprise a high angle boundary which is almost always impossible to measure using Laue methods due to the limited width of the grain, slivers are also considered critical defects. Other grain related linear defects include low angle grain boundaries which are above the allowed limit of misorientation. Non grain related linear defects include linear chains of pores, surface micro-cracks and dross or inclusions which are normally only detectable using Fluorescent Penetrant Inspection (FPI). Another well known potential defect in single crystal and columnar-grained castings is recrystallized grains. Although these develop only during the solution heat treatment and/or reconditioning, repair, rejuvenation treatments, they can be considered casting defects since they are caused by excessive local deformation of the cast article due to the differential thermal contraction of the casting alloy, ceramic core and ceramic shell mold as the casting assembly cools. Recrystallized grains typically occur in the regions of highest deformation which are usually fillets, corners and design features which constrain the core or shell against the cast article.
Those skilled in the art of casting single crystal and columnar grained articles are aware of the economic significance of such linear defects. Part cost decreases substantially when more parts can be cast at the same time in one cluster. However, due to the increased mass of liquid alloy that must be cooled, and the decreased thermal radiation allowed per unit area of shell mould from a denser, heavier cluster to the cooling zone, clusters with more pieces naturally tend to exhibit lower thermal gradients and high inclinations of the solid/liquid interface than clusters with fewer pieces on them. Larger cluster sizes therefore are more freckle prone than clusters with fewer pieces. Even in small sized clusters of castings, freckles are a well known problem as it is desirable to pull castings as quickly as possible into the cold zone, but more rapid withdrawal also results in lower thermal gradients across the solid-liquid interface. Typically, it is industry standard for buyers of single crystal castings to reject some articles based on specifications limiting the acceptable sizes and locations of freckle defects on each casting. The rate of rejection can be anywhere from under 5% to over 50% depending on the alloy used and size of the article. The casting process parameters (including cluster size) are always developed in order to achieve a balance between production rate and the rate of rejection from casting defects to optimize overall process economics. Depending on the alloy chemistry (for example, alloys rich in Ti, Al, W and poor in Ta are more prone to freckles) the optimum process may still produce significant scrap from linear defects. So far no method has been disclosed to repair these defects, but such a method would significantly impact the economics of the columnar grained or single crystal casting process. Parts that are normally thrown away (only value is that of the alloy—about 10% or less of the part value) could be restored to full value for a fraction of the manufacturing cost.
One benefit that is sought after is a method to repair defects in single crystal or columnar grained articles to restore the full strength of the defect-free material without compromising the quality of the material. Until now, no method has been available to carry out such a repair operation. However, newly invented single crystal welding processes offer possibilities.
One such welding process is disclosed in U.S. Pat. No. 6,024,792 in which a powder or wire is fed into a laser beam (or other heat source) as it melts an existing single crystal structure. Another welding method is disclosed in U.S. Pat. No. 6,084,196 using plasma-transfer arc to deposit material into a damaged section of a single crystal article.
EP-A1-0 558 870 describes free form welding of metallic articles with a laser where already built-up material acts as a substrate for newly deposited metal. However, the authors either use powder or wire feed and pulsed laser irradiation. EP-A1-0 740 977 furthermore describes a containerless method of producing crack free metallic articles using a laser beam operating at moderate power density. A large diameter beam produces a shallow melt pool from which single crystal articles are generated by addition of powder. The relatively long interaction time is claimed to be advantageous to reduce the cracks resulting from hot tearing defects during solidification. The method, however, focuses on the generation of new parts. Also the process parameters are chosen in order to reduce thermal gradients and thus stress, which is not favourable for single crystal solidification.
A similar technique is described in U.S. Pat. No. 5,914,059 as a suitable method of repairing metallic articles by energy beam deposition with reduced power density. Again, the focus is on remelting filler material into defective regions of a single-crystal parts and on maintaining process conditions that reduce the cracking risk. The same intention is mentioned in U.S. Pat. No. 6,054,672 which describes laser welding of super alloy articles. Here the strategy is to reduce stress by preheating the entire weld area and the region adjacent to it to a ductile temperature above the ageing temperature but well below the melting temperature of the melting temperature of the super alloy.
U.S. Pat. No. 5,837,960 describes a computer aided laser manufacturing process which is used to generate articles by laser/powder techniques. Again, the addition of powder is an essential part of that invention. U.S. Pat. No. 5,312,584 describes a moldless/coreless method of producing single crystal castings of nickel-aluminides. In this case a laser is used to melt a Ni—Al target which melts, forms a drip and solidifies on an underlying single crystal substrate.
DE-C1-199 49 972 uses a laser method to generate 3D objects using a digitizer/optical vision system and layer by layer material build-up. The method requires additional material supply which is not necessary for the local repair of casting defects.
A method for remedying material defects is suggested in U.S. Pat. No. 4,960,611. Here, however, the laser is used to irradiate small coating defects which are caused by adhesion of dust particles, oil droplets or similar. The laser vaporizes the defects and creates a small cavity which is repaired by addition of filler material and subsequent curing with IR laser radiation.